Electronic test instruments are used to measure the electrical activity of a circuit and to display that electrical activity to a user in a human recognizable form such as a number on a liquid crystal display (LCD) or a representative graph on a cathode ray tube (CRT). For instance, a commonly used piece of an electronic test instrument known as a multimeter may measure and display electrical activity in terms of voltage, current, resistance, frequency, capacitance and inductance.
A multimeter has a pair of test leads that, by convention, includes a red "signal" lead and a black "common" lead. The red lead is coupled to a selector switch that routes a test signal from a circuit under test to an intermediate circuit that normalizes the test signal before being passed to a measurement circuit. To complete the test circuit, the measurement circuit is also coupled to a common ground that is connected to the black test lead. After measuring the test signal, a display circuit translates the measurement into a human recognizable form and displays that result to the user on a display.
The measurement circuit is typically implemented as an integrated circuit having a plurality of external "pins" that conduct the test signal to the appropriate measurement functions within the integrated circuit. Integrated circuits have inherent operating limitations, such as the range of voltage that can be applied to the integrated circuit's pins without damaging the integrated circuit. To accommodate a wide range of test signals, the selector switch directs the test signal through an intermediate circuit that normalizes the test signal so that the same measurement circuit can be used to measure many different signals. A common technique is to have the selector switch implement "ranges" that, depending on the positioning of the selector switch to an indicated range, directs the test signal to an intermediate circuit associated with that range. The display typically includes an indication of the range selected. For example, a 1000 volt test signal could be measured by selecting a 1000 volt range with the selector switch, while a 10 volt test signal could be measured by selecting a 20 volt range with the selector switch.
In addition to translating the test signal into a form usable to the integrated circuit, the intermediate circuit may also have a protective function that keeps damaging test signals from the integrated circuit. The protection provided by the intermediate circuit is usually a resistor connected in series with the intermediate and measurement circuits to restrict the current flow in those circuits. However, it is not uncommon for the test signal to be directed to an inappropriate intermediate circuit. For instance, a user may knowingly or unknowingly connect the test leads to a 1000 volt test signal while the selector switch is inadvertently set to the 20 volt range. This applies a 1000 volt steady state voltage to the components in the 20 volt range intermediate circuit that will likely damage both the components in that intermediate circuit and the integrated circuit. Unexpected events also may direct a damaging test signal to an inappropriate intermediate circuit and the integrated circuit. For example, if the leads are attached to a motor, the test signal may include an "inductive kick" that transmits a high voltage transient through the test leads when the motor first starts. Damaging voltage transients may also appear on power lines, or even be introduced into circuits under test by static electricity.
One common method of providing protection in an intermediate circuit is to place one or more "protective" resistors in series in the test signal path (also called a measurement path). These series resistors dissipate excess voltages as heat in a relationship well known in the art and limit the current that reaches the integrated circuit. The use of protective resistors, however, involves tradeoffs that may effect the accuracy of the measurement of the test signal. For example, the protective resistor's own resistance interacts with resistor's own parasitic capacitance, the parasitic capacitance of the integrated circuit's pins, the capacitance caused by the traces on the circuit board, and other undesirable capacitive coupling, to create a low pass filter that attenuates higher frequency components of the test signal. While a larger protective resistance is desirable because it drops the signal voltage and limits the current to values that are less likely to damage the measurement circuit, the larger the protective resistance, the greater the attenuation of the higher frequencies. (The frequency passing ability of the resistor is inversely proportioned to the resistance of the resistor.) Therefore, prior art methods that use protective resistors require a compromise between the ability of the protective resistor to protect the circuit and the ability to pass an extended bandwidth of test signals.
Examples of methods employed to compensate for the loss of bandwidth caused by adding a protective resistance in series with the test signal path are found in the Fluke 87 and Fluke 8060 digital multimeters manufactured by Fluke Corporation, Everett, Wash. In the Fluke 87, a first test signal path contains a very large value series resistor protecting the measurement circuit. To compensate for the frequency rolloff caused by this large value series resistor, a second, parallel, test signal path is provided that employs a lower value resistor in series with a coupling capacitor that extends the frequency bandwidth of the intermediate circuit by passing some of the higher frequencies to the measurement circuit. This solution provides a greater frequency bandwidth, but has the attendant problems of compensation matching and the introduction of thermal noise caused by the very high value resistor.
In the Fluke 8060 a lower resistance in the intermediate path results in the an extended bandwidth, but at the expense of not providing greater overvoltage protection. A higher value protective resistor is used near the end of the test signal path close to the measurement circuit. The test signal path from the protective resistor to the integrated circuit containing the measurement circuit is then shielded from stray capacitance using a protective voltage that follows the test signal present on the test signal path. While this method extends the bandwidth of the circuit, it comes at the price of overvoltage protection limited to approximately 20 seconds.
It would be desirable to provide a circuit that combines a high level of protection against steady-state overvoltage and voltage transient conditions while maintaining the ability to pass test signals within an extended bandwidth. Preferably, the circuit would be able to withstand a continuous (steady-state) overvoltage for an extended period of time as well as short period voltage transients. The circuit would also preferably isolate the test signal path from the capacitive coupling that would otherwise adversely effect the test signal path's frequency transmission ability. The present invention is directed to such a circuit.